Reflecting the ever-increasing demand for pictures having higher quality, it is required to reproduce the gradation or tone of an original continuous-tone picture correctly in a reproduced picture upon making a print or copy from the original picture or printing out the original picture by means of a printer. The term "original continuous-tone picture" as used herein should be interpreted in a broad sense, so that it may embrace soft originals composed of electronic data from image-related computers, TV sets (including high-definition TV sets), video recorders, electronic still cameras (video floppies), etc., to say nothing of hard originals such as paintings, monochrome pictures, color pictures, handwritten manuscripts, freehand drawings and the like.
The reproduction of original continuous-tone pictures is dealt with in an extremely wide range of industry and technical field as already mentioned above. Many problems however still remain in expressing the tone of an original picture suitably in the tone of a picture to be reproduced, namely, in the reproduction of the tone of the original picture. It is hence the current circumstances that no rational and scientific technique has been established yet for the conversion of a tone.
Taking, for example, printing which is a representative industry and technical field, the abovementioned problems will hereinafter be described.
When a printed picture of a halftone is produced from a photographic original of a continuous tone by using a photomechanical camera or the like or a printed picture of a halftone is produced from a photographic color original by performing color separation of the photographic original by means of an electronic color separation apparatus (monochrome scanner, color scanner), it is indispensable to convert the tone of the original picture from the continuous tone to the halftone upon production of the printed picture as is known very well.
Non-scientific approaches have however been taken in the tonal conversion of pictures in the conventional photomechanical technology [including both photomechanical processes for making original films (e.g., halftone positive films and halftone negative plate. The same definition will be applied hereinafter.) by using photographic techniques and electromechanical processes for making original films electronically]. Namely, no satisfactory scientific analysis and study has heretofore been conducted with respect to such issues as what gradation characteristics a halftone picture for a photomechanical process should have upon production of a printed picture of a halftone from an original continuous-tone picture, how a gradation characteristic curve should be specified as a standard work curve for obtaining such a halftone picture useful in the photomechanical process, and how such a gradation characteristic curve should be obtained rationally. In fact, these issues have heretofore been dealt with depending primarily on the experiences and perception of photomechanical technicians or empirical and personal judgements of design engineers of electronic color separation apparatus (scanners).
The above-mentioned manner of solution still prevails even the present day of wide spread adoption of total scanners, one of very expensive and technically advanced photomechanical equipment, in the present field of art relating to photomechanical processes and printing. Incidentally, a standard work characteristic curve is employed as a standard or reference when a tonal conversion is effected by performing halftone scanning or both color separation and halftone scanning with advanced photomechanical equipment such as monochrome scanner or color scanner. This standard work characteristic curve is a characteristic curve of a halftone picture, which is to be used as a standard or reference upon performing halftone scanning or color separation. It is also called "halftone characteristic curve" in connection with monochrome scanners and "color-separation characteristic curve" in the case of color scanners. Such a standard work characteristic curve is determined by the knowledge of engineers of manufacturers of these equipment. Their knowledge is based on experiences and perception. Under the circumstances, users rely upon a standard work characteristic curve stored beforehand in the memory of their equipment or a standard work characteristic curve determined on the basis of the experiences and perception of the users.
Accordingly, the tonal conversion of the continuous-tone of an original picture into the halftone of a printed picture is not performed rationally and scientifically in the photomechanical technology. This has prevented the rational systematization of the overall photomechanical technology.
From the realities, photomechanical work is carried out by using a variety of advanced electronic color separation apparatus (scanners) in order to produce printed pictures. The photomechanical work is however still accompanied by problems such as those to be mentioned below.
(i) Problems pertaining to the tonal expression of intermediate tone:
(a) In spite of today's wide-spread adoption of advanced photomechanical equipment, it is difficult to achieve the gray balance (density balance) of an intermediate tone. The term "intermediate tone" as used herein means the tone of a halftone area, whose dot area percentage ranges from about 40% to about 80%, on a print. PA1 (b) The color of the intermediate tone is dull and does not have any vivid tone. PA1 (c) It is difficult to emphasize or stress the intermediate tone. PA1 (d) The delicate gradation of the intermediate tone (which is of course related closely to the gradation and tone of the entire pattern) can not be reproduced well. PA1 About 10% in color-separation work where usual quality is required. PA1 About 20%-30% in color-separation work where high quality is required. PA1 y: the halftone intensity of the corresponding control point Y on the picture to be reproduced, such as a printed picture, as expressed in terms of dot area percentage or the like; PA1 y.sub.h : a desired halftone intensity set for the brightest area of the picture to be reproduced, such as the picture to be printed, as expressed in terms of dot area percentage or the like; PA1 y.sub.s : a desired halftone intensity set for the darkest area of the picture to be reproduced, such as the picture to be printed, as expressed in terms of dot area percentage of the like; PA1 .alpha.: the reflectivity of a base material on which the halftone picture is to be reproduced; PA1 .beta.: the surface reflectivity of a medium for visualizing the halftone picture; and PA1 k: the ratio of the density range of the halftone picture to be reproduced to the density range of the original continuous-tone picture.
Reflecting the above problems, re-scanning is required to the following extents in actual work.
It is the poor color balance of the intermediate tone in the printed picture that accounts for a major part of causes for the re-scanning. Under the circumstances, the poor color balance amounts to about 30% -40% of the whole causes.
(ii) Problems relating to the preparation of a standard work characteristic curve for processing a non-standard color original:
A color separation characteristic curve (i.e., tonal conversion curve), which is a standard work characteristic curve required for the production of a printed picture having desired quality, must be determined relying upon the experiences and perception of workers, for example, when it is desired to perform color separation on a non-standard color original, namely, a non-standard color original having quality other than that of standard color originals taken into consideration upon designing a color scanner or the like (i.e., an original not subjected to proper exposure and/or development) or when it is desired to perform color separation similarly by using a color separation characteristic curve other than that stored beforehand in a color scanner of the like. Further, tremendous time, cost, labor and experiences are also required upon inputting basic data in a color scanner in order to obtain such a characteristic curve.
In the case of a non-standard color original containing a color fog in the high-light area thereof, its color separation work is confused so that the quality is not stabilized. Such non-standard originals now account for as much as 30-40%.
(iii) Operational problems of scanners:
(a) A very long period of time is required until an operator learns the operation technique of a scanner, receives training on the scanner, and then masters it well.
(b) The product quality varies when the technician or operator is replaced, whereby the product quality can hardly be unified and stabilized.
(c) The product quality changes considerably depending on the place of work or the operator, even when the same scanner is used.
(d) The high performance of a scanner may not be fully utilized in many instances.
(e) It becomes difficult to make products of the same quality when an original is replaced by another original having different quality or contents.
(f) It is very difficult to make a product whose quality meets the customer's desire.
(g) It is difficult to unify the quality of products when a great deal of originals are received at once and their processing must be performed at plural work places.
(h) Mutual understanding is not achieved well between scanner manufacturers and those dealing with plate-making and printing, regarding the equipment and the quality of products.
(iv) Lack of scientific approach for the production of halftone pictures suitable for use in the photomechanical process:
(1) Further, design engineers of electronic color separation apparatus (scanners) and the like do not have full understanding of plate-making and printing techniques. This lack of understanding has resulted in the lack of flexibility in work by the apparatus, thereby failing to solve various problems such that diversified needs of print orderers can hardly be met, photomechanical engineers have difficulties in experimenting and developing new creative color separation methods, and jumping takes place in the gradation of a halftone picture in spite of the use of the apparatus.
(2) As a more fundamental problem, it has not been attempted to establish a universal and rational, tonal conversion method for the tonal conversion of an original continuous-tone picture into a printed halftone picture, which tonal conversion is indispensable for the production of the printed picture. For this reason, various confusions have been induced unnecessarily regarding tonal conversion not only in the photomechanical business but also in the printing business and even on the side of orderers for prints.
The picture forming technique described above placing emphasis on printed pictures will next be summarized from the viewpoint of picture or image processing techniques including other techniques for the formation of pictures.
Picture processing techniques adapted to form reproduced pictures such as binary pictures and multivalued pictures from continuous-tone pictures include indirect picture processing methods for obtaining a reproduced picture from pictorial information of an original via an intermediate soft picture such as color separation characteristic curve or halftone characteristic curve as well as direct picture processing methods for obtaining a reproduced picture directly from pictorial information of an original. A typical example of the former methods is a scanner-dependent picture processing method employed upon production of a printed picture, while representative examples of the latter methods include the picture processing method for obtaining a CRT picture for a color separation monitor, a heat-sensitive transfer picture, a reproduced ink-jet picture, a reproduced toner picture or the like.
It is common to these picture processing methods that electronics, computer, mechatronics and the like are employed as technical means and an analog processing method and/or digital processing method is relied upon as a processing method of pictorial information.
However, none of these picture processing methods and picture processing techniques includes any scientific and rational technique for the tonal conversion of pictures. Most of the picture processing techniques described above have a function to control the tone of a picture and the luminance of each picture element. However, their controls are, as a matter of fact, dependent heavily on the experiences and perception of a man.
The adjustment of the tonal conversion of a picture on the basis of the experiences and perception of a man is in fact the fundamental cause for the various problems which arise upon formation of a reproduced picture such as binary picture or multivalued picture from an original continuous-tone picture. The above adjustment is a fundamental matter which should be rationalized as soon as possible in the present picture processing techniques.